Radio frequency amplifiers with the common industrial frequencies of 13.56 MHz and 27.12 MHz and output powers of 1 kW to 50 kW are conventionally used in the field of laser excitation or plasma processes.
Load impedances of laser excitation or plasma processes can be non-linear and dynamic, exhibiting unpredictable changes. These dynamic impedance changes may generate reflections that produce losses in the radio frequency amplifier. Large reactive energies that are stored in the reactive elements of the radio frequency amplifiers, in the feed lines, and in the reactive elements of matching networks can thereby be discharged. Such discharge may generate high voltages or currents, may excite oscillations in the radio frequency amplifier, or may destroy components of the radio frequency amplifier. Such load impedance changes may occur, for example, during striking of the laser excitation or plasma processes, or during arcing in the plasma process.
Radio frequency-operated laser excitations and, to an increasing extent, also radio frequency-excited plasma processes can be operated in a pulsed manner, i.e., the radio frequency amplifiers are switched on and off with pulse frequencies of, for example, 100 Hz to 300 kHz, or are switched between two power ranges. Temporary reflections may be produced during each switching process, and these temporary reflections may be converted into lost energy, that may accumulate as excess heat in the radio frequency amplifiers.
The output stages of such radio frequency amplifiers may be realized with transistors for small powers (1-6 kW). Alternatively, for larger power, tubes may be used as output stages of radio frequency amplifiers. Tubes can be more robust to unwanted reflections and can dissipate lost energy better than transistors. Tubes can be, however, more expensive than transistors, can be subject to wear during operation, and can be relatively large. Tube radio frequency amplifiers can be bundled together with a drive circuit and a cooling system in switching cabinets of a size of approximately 0.8 m×1 m×2 m.